Increased adult weight gains have consistently been shown in the published literature to increase the risk of postmenopausal breast cancer. To date no published research has examined weight cycling (i.e. substantial sequential positive and negative weight fluctuations) and risk of postmenopausal breast cancer. Neither is there any consensus on how to measure weight cycling. Weight cycling is one of the very few potential risk factors that can be modified over a woman's lifetime. Given that weight cycling has a biologically plausible association with this disease, the examination of weight cycling and postmenopausal breast cancer may have public health importance. Outpatient records from Kaiser Permanente Southern California (KPSC) offer a unique opportunity to retrospectively examine relatively dense weight measurements and confounders from the 1950's to present. KPSC has 3.3 million members and reported approximately 4500 breast cancer diagnoses from 1994 through 1996. The present proposal is an age and time-of-enrollment matched case- control study of 500 cases and 500 controls. The Primary Objective is to determine if weight cycling, using various operational definitions while controlling for other risk factors, increases the risk of postmenopausal breast cancer. Specific Aims are 1) to evaluate if an increased measure of weight cycling from age 18 increases the risk of postmenopausal breast cancer, and 2) to apply various operational definitions of weight cycling to examine agreement and to compare strengths of association with the risk of postmenopausal breast cancer.